1. Field of the Invention
This invention relates generally to water reducing chemical admixtures for concrete, and particularly to lignin based additives used wholly as, or as a component in, a water reducing chemical admixture.
2. Description of the Related Art
In the construction industry water reducing admixtures are used, in general, to increase the strength of fresh concrete by lessening the amount of water required to achieve a certain fluidity. They are also used to increase the fluidity of fresh concrete, and make concrete of equal strength less costly by removal of a portion of the cement.
Lignosulfonate is a common cement dispersant. A large portion of the world's production of lignosulfonate is used as water reducing admixtures to increase the strength of concrete in various construction applications. When used in cement compositions lignosulfonate reduces the amount of water required, and improves the workability of the cement. Lignosulfonate, however, has several negative consequences associated with its use, including retarding the set of fresh concrete and increasing the amount of entrapped air present in concrete. The former is undesirable because delayed setting slows production rates, and the latter is undesirable because of decreased strength of the set concrete.
Typically, these shortcomings of lignosulfonate encountered during their use in cement compositions are mitigated through formulation of the admixture. For example, accelerators are added to admixture products to offset the retarding effect. The reagents commonly used as accelerators include the calcium salts of chloride, nitrate, nitrite, formate and thiocyanate. Inherently, however, these materials add cost to the admixture. Another common formulation practice involves the use of defoamers. Various defoamer products have been used with some success to lower the entrapped air caused by lignosulfonate. Unfortunately, these defoamers have a negative impact on air entraining admixtures that are employed to improve the freeze-thaw durability of concrete. Therefore, addition of defoamers is not an accepted practice where freeze-thaw durability is required.
In the construction industry various synthetic dispersants are employed that do not suffer from the above-listed drawbacks associated with lignosulfonates. Polynaphthalene sulfonate, melamine resin, and carboxylic polymers are, generally, the three classes of anionic dispersants used for this application. These materials tend not to retard the set of concrete, and to result in only low quantities of entrapped air. However, they are derived from petroleum resources, and are much more costly than lignosulfonate. It would be desirable to have a cement dispersant without the negative side effects of set retardation and air entrainment, but derived from an environmentally benign, readily available, and relatively inexpensive source. Lignosulfonate, which is produced in large quantities as a byproduct of the pulp and paper industry, has the potential to be such a dispersant, if it can be suitably modified to overcome its shortcomings in cement compositions.
In attempts to minimize the set-retarding and air-entraining effects of lignosulfonate, some modifications to lignosulfonate that have been employed in the past include:
1. Alkali air oxidation (Japanese Kokai Sho-No. 55-56051) PA1 2. Ultrafiltration (GB No. 2092564) PA1 3. Polymerization of lignosulfonate to increase molecular weight (Japanese Kokoku Sho-No. 58-176158) PA1 4. Co-polymerization of lignin and lignosulfonate with various vinylic monomers (U.S. Pat. No. 4,977,227)
Generally, these modifications have been found either to have a minimal impact on the final performance of the lignosulfonate, or to be cost prohibitive.
It has been shown that the addition of nitrogen functionality to lignosulfonate will decrease the setting time of various cement compositions, as disclosed in the U.S. Pat. No. 4,990,191. In this patent, the method of nitrogen incorporation is through the reaction of lignosulfonate with a polyamine and an aldehyde, also known as the Mannich condensation (U.S. Pat. No. 4,781,840).
Yet another already disclosed method for introducing nitrogen functionality in lignin and lignosulfonate utilizes a reaction with ammonia and an oxidant, preferably air or oxygen-containing gas mixtures. The process is known as ammoxidation, and the use of ammoxidized lignosulfonate has been disclosed for slow nitrogen release fertilizers. U.S. Pat. No. 5,720,792 discloses the latest embodiment of this technology.